It's unfortunate, but Fx v57 will sound the death knoll of many extensions - FEBE included. Most of the extensions I have written simply cannot be re-written using the WebExtensions platform. In particular, FEBE, CLEO, OPIE, JOE, and FECT require access to regions of the Fx browser internals that WebExtensions does not (and apparently will not ever) provide.

In the future, only Pale Moon (and other Fx forks that still use XPCOM and XUL) will be supported..